Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction.
In a common operating mode, multimedia projection systems receive analog video signals from a personal computer ("PC"). The video signals may represent still, partial-, or full-motion display images of a type rendered by the PC. The analog video signals are typically converted in the projection system into digital video signals that control a digitally driven image-forming device, such as a liquid crystal display ("LCD") or a digital micro mirror device ("DMD").
A popular type of multimedia projection system employs a light source and optical path components upstream and downstream of the image-forming device to project the image onto a display screen. An example of a DMD-based multimedia projector is the model LP420 manufactured by In Focus Systems, Inc., of Wilsonville, Oreg., the assignee of this application.
Significant effort has been invested into developing projectors producing bright, high-quality, color images. However, the optical performance of conventional projectors is often less than satisfactory. For example, suitable projected image brightness is difficult to achieve, especially when using compact portable color projectors in a well-lighted room.
Because LCD displays have significant light attenuation and triple path color light paths are heavy and bulky, portable multimedia projectors typically employ DMD displays in a single light path configuration. Producing a projected color image with this configuration typically requires projecting a frame sequential image through some form of sequential color modulator, such as a color wheel.
The use of color wheels in frame sequential color ("FSC") display systems has been known for many years and was made famous (or infamous) in early attempts to develop color television sets. With technological advances, however, color wheel display implementations are still useful today.
FIG. 1 shows a typical prior art FSC display system 10 in which a sensor 12 senses a timing mark 14 to detect a predetermined color index position of a motor 16 that rotates a color wheel 18 having respective red, green, and blue filter segments R, G, and B. A light source 20 projects a light beam 22 through color wheel 18 and a relay lens 24 onto a display device 26, such as an LCD-based light valve or a DMD. A display controller (not shown) drives display device 26 with sequential red, green, and blue image data that are timed to coincide with the propagation of light beam 22 through the respective filter segments R, G, and B of color wheel 18. Clearly, successful operation of a FSC display system depends on properly synchronizing the red, green, and blue image data to the angular position of color wheel 18.
Sensor 12 typically employs any of optoelectrical or electro mechanical shaft position or motor armature position detectors and usually requires some means for aligning timing mark 14 to the start of one of the filter segments. This alignment is typically a costly and error prone mechanical adjustment that accounts for angular differences between motor 16 and the mechanical mounting of filter segments R, G, and B. Of course, electrical or mechanical delays associated with sensor 12 further contribute to alignment errors.
The accumulated angular errors open the possibility of synchronization errors between the red, green, and blue image data to the angular position of color wheel 18, a possibility that prior workers avoided by building a timing duty cycle into the display controller electronics. The timing duty cycle provides for driving display device 26 with the red, green, and blue image data for only a portion of the time when light beam 22 is propagating through each of respective filter segments R, G, and B, thereby preventing illuminating display device 26 with an improper color. Unfortunately, the timing duty cycle reduces the total amount of illumination available for displaying each color and, therefore, reduces the brightness of the resultant displayed color image.
A solution for minimizing color wheel synchronization errors is described in copending U.S. Pat. application Ser. No. 09/136,799, filed Aug. 19, 1998, for COLOR WHEEL SYNCHRONIZATION APPARATUS AND METHOD, which is assigned to the assignee of this application. FIG. 2, which is duplicated herein, shows a multimedia projector 30 in which a light source 32 emits polychromatic light that propagates along a folded optical path 34 through projector 30. Light source 32 is a metal halide arc lamp 36 with an integral elliptical reflector 38. Optical path 34 includes a condenser lens 40, a color wheel 42, an airspace doublet lens 44, a fold mirror 46, a relay lens 48, a display device 50, and a projection lens 52. The optical components are held together by an optical frame 54 that is enclosed within a projector housing (not shown). A display controller 56 receives color image data from a PC 58 and processes the image data into frame sequential red, green, and blue image data, sequential frames of which are conveyed to DMD 50 in proper synchronism with the angular position of color wheel 42. A power supply 60 is electrically connected to light source 32 and display controller 56 and also powers a cooling fan 62 and a free running DC motor 64 that rotates color wheel 42. Display controller 56 controls display device 50 such that light propagating from relay lens 48 is selectively reflected either toward projection lens 52 or toward a light-absorbing surface 66 mounted on or near optical frame 54.
DC motor 64 rotates color wheel 42 at about 6,650 to 7,500 rpm. Color wheel 42 includes color filter segments R, G, and B that each surround about 120 degrees of color wheel 42. Color wheel synchronization is achieved by detecting which color filter segment is in optical path 34 and for how long. Particular colors of light propagating through color wheel 42 are sensed to generate synchronization timing data. In particular, a color selective light sensor 68 is positioned off optical path 34 and adjacent to relay lens 48 to receive light scattered off fold mirror 46, a position that does not intercept any ultimately projected light. Light source 32 has sufficient intensity to allow receiving scattered light at various locations within optical frame 54.
However, the applicant has discovered that the above-described color wheel synchronizing solution is not suitable for use in certain multimedia projectors. For example, to achieve form factor goals and increase output lumens, some multimedia projectors pass light through light integrating devices, such as a solid glass integrator rod, which minimizes or eliminates extra unused light downstream of the color wheel. Accordingly, there is no convenient place to position a photo detector without robbing the DMD of lumens. The above-described photo detector also protrudes into an area adjacent to the light path, rendering it vulnerable to breakage. Furthermore, the photo detector is connected to the display controller by a cable and connector assembly that is costly and requires undue assembly. Moreover, the photo detector senses the color change at the same time as the DMD, which requires liming adjustments to ensure correct synchronization of color wheel color changes and the frame sequential color images driving the DMD.
What is needed, therefore, is a color wheel synchronization technique that circumvents the above-described problems and substantially eliminates any mechanical, optical, and electrical rotational timing errors that are intrinsic to prior color wheel systems.